The present invention relates to athletic racket string construction and, more particularly, to a synthetic racket string construction having a seven strand core and an outer layer of wrapping fibers, especially ally in oversized racket applications where the string diameter is between 1.26 mm and 1.45 mm to achieve the desired strength.
The earliest strings used in tennis rackets was natural gut made from animal fibers. Tennis players found and still find natural gut to be acceptable because of its good tensile modulus which produces excellent elasticity and resilience. The good tensile modulus of natural gut has been attributed, at least in part, to its multi-fiber structure.
However, natural gut is not without disadvantages, and these disadvantages discourage its wide use. For instance, natural gut is not humidity proof. On one hand, it becomes sticky at its surface and stretches easily when wet. On the other hand, natural gut contracts when it is dry. Thus, it will be readily appreciated that a natural gut strung in a racket frame when stretched under high humidity conditions may break under dry conditions due to contraction.
Natural gut also frays easily and is not particularly durable. It has a relatively low breaking strength with only little room for improvement. The sources of animal fiber are also becoming limited; in turn, this results in higher prices. As shown in U.S. Pat. No. 1,669,212, attempts were made to improve the performance of natural gut by spirally winding it around a rubber core with such tightness as to hold the core under a certain amount of initial compression to be augmented when the string is tensioned to increase the resiliency lengthwise of the string and increase elasticity transversely of the string.
Given the tremendous rise and popularity in racket sports such as tennis, racketball and squash, it was inevitable that racket manufacturers would turn to an alternative and cheaper substitute. The development of synthetic fiber caused a large scale use of synthetic fibers in rackets. For background purposes, synthetic string suitable for this purpose can be generally grouped into two categories, i.e., a string with a center core and a string without a center core.
The center core string primarily consist of a large diameter core wrapped with smaller diameter fibers. The core diameter usually ranges from 0.8 mm to 1.0 mm depending on the desired string guage. The wrapped fibers have a diameter of between about 0.16 mm and 0.25 mm. The manner in which the fibers are wrapped around the core, as well as the materials used for the center core and the wrapping fibers, will vary depending on the desired characteristics of the string.
The center core synthetic string offers the advantages of being humidity proof, durable and relatively inexpensive. Nevertheless, it has been found that one problem with this type of string is that, due to a larger than desirable cross section for the center core the string becomes relatively stiff. Furthermore, it is known that the tensile strength per unit cross section decreases when the cross section of a synthetic fiber exceeds a certain value, the value depending on the fibers involved. In general, however, when the diameter exceeds 0.40 mm to 0.60 mm, the tensile strength per unit cross section decreases fairly rapidly. Thus, manufacturers are forced to use larger cross section for the center core to obtain the necessary total tensile strength sufficient for tennis rackets, particularly oversized tennis rackets. This has the unfortunate effect of increasing the stiffness of the string and lowering the playability of the racket even though the string becomes more durable in the process.
To avoid the foregoing disadvantages of the center core synthetic string, it has been proposed to use a string without a center core. To this end, a bunch of fibers are twisted together and then properly treated at the surface of the bunch of fibers. I believe it is fair to say that the choice of the number of fibers to be twisted together is usually rather arbitrary. For example, it has been known to use between 51 to 53 fibers in a string without a center core. Some manufacturers use as many as 1400 twisted fibers.
It has been found that if too many fibers are twisted together, the string becomes easily frayed as is the case with natural gut. More. over, it is hard to twist too many fibers together in a geometrically stable fashion. For instance, sometimes cavities result among the twisted fibers, and this makes the string easier to fray, thereby decreasing durability of the string. By the same token, if too few fibers are used, e.g. three, the desired characteristic features of natural gut string will be missing.
Various examples of prior art string are shown in U.S. Pat. Nos. 1,669,212; 2,181,475; 3,329,061, 3,745,756; 4,016,714; 4,084,399; 4,159,618, and 4,275,117.